1. Field of the Invention
The present invention relates to an image heating apparatus preferably used as a toner image fixing apparatus of an image forming apparatus such as a copier, a laser beam printer, and a facsimile machine.
The image heating apparatus referred to herein includes not only a fixing apparatus for heating and fixing an unfixed image onto a recording material as a permanent image but also an apparatus for temporarily fixing an unfixed image onto a recording material and an apparatus for improving the quality of the surface nature of image such as gloss by reheating a recording material carrying an image thereon.
2. Description of the Related Art
There is an image forming apparatus in which an unfixed toner image corresponding to image information as an object is formed and borne on a surface of a recording material with toner (developer) by direct or indirect transfer methods by means of an appropriate one of image forming process means. Examples of the image forming process means include electrophotography, electrostatic recording, magnetic recording, and the like. It should be noted that the toner is made of hot-melt resin and the like. Examples of the recording material include paper, printing paper, a transfer material sheet, an OHT sheet, gloss paper, a gloss film, electrofax paper, electrostatic recording paper, and the like.
The above image forming apparatus includes a fixing apparatus serving as an image heating apparatus for heating and fixing an unfixed toner image onto a surface of a recording material as a permanently-fixed image. A heat roller-type fixing apparatus and a film (belt)-type fixing apparatus are often used as the fixing apparatus. The film (belt)-type fixing apparatus has a shorter warm-up time than the heat roller-type fixing apparatus, and operates on demand and is cheaper. It should be noted that the warm-up time is a time from when the fixing apparatus begins driving or from when the fixing apparatus is in standby state to when the fixing apparatus reaches a temperature needed for heating/fixing processing.
A so-called “non-sheet-feeding portion temperature rise” phenomenon occurs in the above fixing apparatus. This is a phenomenon that occurs when the fixing apparatus is fed with a small size recording material having a smaller sheet width than a large size recording material having the maximum sheet width that can be fed in the fixing apparatus, and this phenomenon occurs for the following reasons. In a non-sheet-feeding portion of a fixing nip portion corresponding to a differential portion of sheet width between the large size recording material and the small size recording material, heat is not consumed for heating the recording material but is only accumulated, so that the amount of accumulated heat in the non-sheet-feeding portion of the fixing nip portion increases as the small size recording material is successively fed. Accordingly, the temperature of a sheet-feeding portion of the fixing nip portion is adjusted to and maintained at a predetermined fixing temperature, whereas the temperature of the non-sheet-feeding portion increases to a temperature higher than the predetermined fixing temperature. At this occasion, the sheet width of the recording material is the size of the recording material in a direction perpendicular to the conveyance direction of the recording material on the plane of the recording material.
When the above non-sheet-feeding portion temperature rises to a temperature higher than an allowable temperature, a thermal damage trouble occurs on constituent members of the fixing apparatus, which deteriorates the durability. Alternatively, in a case where a large size recording material is fed when the non-sheet-feeding portion temperature excessively rises in the fixing apparatus due to successive feeding of small size recording materials, an end portion hot offset and the like may occur due to an excessively high temperature in the non-sheet-feeding portion temperature rise region of the fixing nip portion.
In order to avoid the problem caused by the above non-sheet-feeding portion temperature rise, the fixing apparatus is configured to be provided with a temperature detection element that detects the temperature of the non-sheet-feeding portion. For example, Japanese Patent Application Laid-Open No. 2001-282036 discloses that the temperature detection element is arranged to detect the temperature of the non-sheet-feeding portion, and when the non-sheet-feeding portion temperature detected by this temperature detection element exceeds a predetermined allowable maximum temperature, the control is performed as follows. That is, sheet feeding operation and image forming operation of an image forming apparatus is controlled to be in a temporary halted state, cooling operation is performed, and when the above non-sheet-feeding portion temperature detected by the temperature detection element decreases to a temperature equal to or less than a certain value, the control is performed so as to resume the halted image forming operation. On the other hand, for example, Japanese Patent Application Laid-Open No. H11-002988 discloses that a temperature control is performed using a temperature detection element of a non-sheet-feeding portion during warm-up process of a fixing apparatus, and when the temperature detection element of the non-sheet-feeding portion reaches a predetermined temperature, the temperature control is performed using a temperature detection element of a sheet-feeding portion. The configuration of Japanese Patent Application Laid-Open No. H11-002988 prevents an increase in the warm-up time.
Since an image to be fixed by the image heating apparatus resides on a recording material, it is necessary to control the temperature of the fixing apparatus based on the temperature detection element of the image region (sheet-feeding portion) in order to heat and fix a high quality image.
Now, the problem to be solved by the present invention will be explained using FIG. 13 and FIG. 14. FIG. 13 and FIG. 14 are time charts for explaining the temperature control of the fixing apparatus. When the temperature control is performed based on the temperature detection element of the sheet-feeding portion, and the warm-up process starts from a state in which the fixing apparatus is sufficiently cooled down, the temperature reaches a target temperature Tt while a difference between a temperature T1 of the sheet-feeding portion (thick solid line) and a temperature T2 of the non-sheet-feeding portion (thin solid line) is still small as shown in FIG. 13. Since the temperature control close to the target temperature Tt can be performed, the above-described problem caused by the non-sheet-feeding portion temperature rise does not occur.
However, after small size recording materials are successively fed, the temperature of the non-sheet-feeding portion becomes higher than that of the sheet-feeding portion. When the temperature control is performed based on the temperature detection element of the sheet-feeding portion at this occasion, the temperature T2 (thin solid line) of the non-sheet-feeding portion becomes higher as shown in FIG. 14, which causes a problem of temperature increase in the non-sheet-feeding portion as described above. In general, a fixing apparatus has an element (such as a safety circuit) to stop heating operation when the temperature reaches a predetermined temperature or more in order to prevent an excessively high temperature. When a detection result of the temperature detection element of the non-sheet-feeding portion indicates that the temperature has reached a temperature Ts at which the safety circuit is activated, there may occur a problem in that the operation of the fixing apparatus is stopped by the activation of the safety circuit at a point A in FIG. 14.
However, the temperature control using the temperature detection element of the non-sheet-feeding portion entails stopping of image forming operation, and accordingly, it takes a longer time to fix an image. Therefore, the configuration of always performing warm-up operation using the temperature detection element of the non-sheet-feeding portion can avoid the problem caused by the non-sheet-feeding portion temperature rise but still has a problem in that it takes a longer time to fix an image.